MYRTLE BEACH, S.C. - Hurricane Florence will begin to lash the Carolinas with powerful winds and violent surf Thursday in what is shaping up as the most dangerous tropical cyclone to hit the region in a generation. Forecasters warn that when it nears land, it could shift to low gear and then meander unpredictably along the coast, sucking energy from the warm ocean as it pounds coastal communities.

Florence's maximum sustained winds weakened modestly Wednesday, taking it down to a Category 3 hurricane, but it simultaneously expanded in size. Its hurricane-force winds extend 70 miles from the center of the eye, and its cloud field is four times the size of Ohio. On Wednesday, a satellite detected an open-ocean wave generated by Florence that appeared to be at least 50 feet high.

The National Hurricane Center on Wednesday tweaked the projected track for Florence to show a turn to the southwest when the storm nears Cape Fear in extreme southeast North Carolina. Florence could potentially drift toward South Carolina while remaining just offshore, as if looking for a port. A long stretch of the Carolinas remains inside the storm track's "cone of uncertainty."

"It could sit there as a Category 3 or 4 hurricane right offshore for a day," said Brian McNoldy, a senior research associate at the University of Miami Rosenstiel School of Marine and Atmospheric Science and a contributor to The Washington Post's Capital Weather Gang. "That would not be good. If it gets close to the coast and just hits the coast or is just slightly inland, but then just sits there, it's like pressing pause at the most violent part of the landfall."

The storm might even reverse direction early next week and head north as a weakened but soggy system dropping rain on already saturated Virginia, West Virginia, Maryland, the District of Columbia and Pennsylvania. These areas are vulnerable to flooding and downed trees after heavy rains this summer.

The forecast calls for 20 to 30 inches of rain in parts of coastal North Carolina, with localized amounts of 40 inches or greater. The hurricane could generate a storm surge as high as 13 feet.

Here in Myrtle Beach, Sandra Lopez-Garcia can't believe her bad fortune. She survived Hurricane Maria when it hit her hometown of Bayamon, Puerto Rico, a year ago. She rode out the storm in her concrete home there, sheltering in a bathroom with her therapy dog.

Now she's in an emergency shelter - Conway High School - just outside of this South Carolina city.

"Because I would like to share with my people the anniversary. They'd been hungry for so long," she said, her voice shaking. "Our island is not the same. We don't have leaves on trees yet."

Lopez-Garcia said she's confident that local and state government officials have adequately prepared for Hurricane Florence. But when asked about President Donald Trump's assurance that the federal government is "totally prepared," she responded with open-mouth shock and shook her head.

Florence is poised to be the most devastating storm to hit this part of the coast since Hurricane Hugo in 1989. The National Weather Service office in Wilmington, North Carolina, wrote that this is likely to be the "storm of a lifetime" for stretches of the Carolinas, "and that's saying a lot given the impacts we've seen from hurricanes Diana, Hugo, Fran, Bonnie, Floyd, and Matthew."

The uncertainties gnaw at government officials who must decide where to order evacuations and where to dispatch emergency responders. The result can be mixed signals, as was the case here in South Carolina, where on Tuesday Gov. Henry McMaster, R, canceled mandatory evacuation orders for several counties along the state's southern coast. But then came the Wednesday revision in the storm track.

"As we have been predicting, this hurricane is unpredictable," McMaster said at a briefing. He said residents in low-lying areas should still leave, even without mandatory evacuation orders, and encouraged those already under such orders to flee.

"Once that hurricane hits, once those high winds get here . . . it will be very difficult if not impossible for anybody to come rescue you if you are in harm's way in one of those zones," he said.

That was echoed Wednesday by officials in Beaufort County, home to Hilton Head Island.

"Now is the time to move," Lt. Col. Neil Baxley of the Beaufort County Sheriff's Office said. He said that residents normally would evacuate to central South Carolina but that this time people should flee to the states to the south.

"Georgia and Florida, wide open, sunshiny, good shape," Baxley said.

But meanwhile, Georgia Gov. Nathan Deal, R, declared an emergency in all of his state's 159 counties, citing the revised forecast of the storm.

Aside from the punishing winds that could strafe the coast for more than a day, the rains expected after landfall could inundate a massive area and trigger widespread flooding. Duke Energy estimated that as many as 3 million people could lose power in the Carolinas because of the storm.

"This is no ordinary storm, and people could be without power for a very long time," said David Fountain, president of Duke Energy North Carolina. "Not days but weeks. We won't even be able to get to some areas for several days."

Hurricane science, never exact, has improved dramatically in recent years, but storms still behave whimsically. Florence is riding along the south side of a high pressure system in the upper atmosphere. There is a second high pressure system further west. In effect, one system escorting the storm to land is going to hand the storm off to the next.

"You can start from the same set of initial conditions, and little changes can make the systems go off into a completely different direction," said Rob Young, a professor of coastal geology at Western Carolina University. "That's chaos theory."

Jeff Byard, an associate administrator at the Federal Emergency Management Agency, said his agency has added resources to South Carolina because of the slight change in the storm's predicted track, but FEMA is keeping in place the resources already deployed in North Carolina and Virginia. He referred to Hurricane Matthew, which wandered up the southeast coast in 2016, causing flooding from which some areas have not recovered, as a potential model.

"One of the lessons learned going back to Hurricane Matthew: We can't necessarily chase the storm," Byard said.

Officials tend to err on the side of caution when deciding whether to order evacuations, said Kieran Shanahan, former secretary of North Carolina's Department of Public Safety, balancing the risks of moving millions of people with leaving them in a storm's potential path.

"Better to come back and clean up after the storm, cleaning up debris, than having to deal with human injury and fatalities," Shanahan said.

Hundreds of thousands of people have heeded the call to evacuate, including the residents of an elegant bungalow in historic New Bern, North Carolina, who wrote on the plywood covering their windows "Just say no 2 Flo."

By Wednesday morning, Myrtle Beach was a virtual ghost town but for police cars. Amusement parks were closed and boarded up. Most of the businesses were shuttered. Gas pumps were wrapped in plastic.

On the beach, Gabrielle Clarke took a final look at the deceptively calm waters. She and her boyfriend would soon drive home to Columbia, South Carolina, about 21/2 hours west. They came to Myrtle Beach on vacation a week ago with no idea that a storm might blow in. Her boyfriend, Lorenzo Brown, thinks local officials reacted too quickly.

"They got ready a little too early," he said, noting that all the grocery stores had closed. "People still need to buy food. We can't eat, so we got to go."

Tonya Eclebery was also taking one last look at the beach as she walked her 1-year-old Yorkie, Chloe, by the water. She said she and her husband plan to ride out the storm, hunkered down at Holiday Sands North hotel, right off the beach, where her husband works as a maintenance employee and where they have a supply of canned food and water. They live in a trailer nearby but decided the concrete hotel overlooking the beach is safer.

"We don't want to be stuck out of the area for two weeks," said Eclebery, a housekeeping supervisor. "We got to go to work."

At the Food Lion in Kitty Hawk, North Carolina, one state to the north, heavy metal storm shutters covered the windows, and a few shoppers pushed overloaded carts along the almost-empty aisles.

"My family all wanted us to leave," said Robin Anderson, who lives in nearby Kill Devil Hills with her husband, two dogs and two cats. "But there's nowhere to go. Even if you can find a hotel, can you afford it?"

At the butcher's counter, Eugene McGirt picked out a package of chicken drumsticks to cook on the grill in case the power goes out. The construction worker, here to help build the Bonner Bridge that links two of the skinny strips of sand that constitute the Outer Banks, used to live in Miami. "I figure I can ride this one out," he said.

Nearby, an older couple in T-shirts consulted a shopping list, pondering a key question: One bottle of margarita mix or two? Two, they decided. Because no one knows how bad Florence will turn out to be.

This article was written by Kristine Phillips, Mark Berman, Sarah Kaplan and Joel Achenbach, reporters for The Washington Post. The Washington Post's Lori Rozsa, Rachel Siegel, Ann Gerhart and Jason Samenow contributed to this report.